Life Cycle Optimization for Residential Air Conditioning Replacement

The electricity consumed by central air conditioners (CACs) currently accounts for 14% of residential electrical use in the United States (DOE, 2001). Population growth in the south (Census Bureau, 2005) and installation of air conditioning in about 90% of all new home construction (Census Bureau, 2006) has the potential to significantly drive up electrical usage. Furthermore, over the coming decades, climate change will not only increase cooling loads across the country (Considine, 2000), but it also stands to increase air conditioning market saturation (Sailor & Pavlova, 2003).

Utilizing more efficient air conditioning units is one strategy to curb energy consumption. In January 2006, the Department of Energy mandated all new manufactured air conditioners have a minimum Seasonal Energy Efficiency Ratio (SEER) rating of 13, which was a 30% increase in energy efficiency over the previous standard. The department estimated that the new standards will save about 4.2 quadrillion BTUs of energy and $1 billion in operating costs over 25 years (DOE, 2005).

However with any product system, the total environmental burden is the accumulation of the environmental burdens associated with the creation, use, and retirement of that product. With many products such as air conditioning, the burden associated with product use, in terms of energy consumed and emissions generated, is substantial. In cases like this, the potential benefit from replacing the product with a more efficient product must be weighed against the burden associated with the creation of a new unit and disposal of the old unit. Just as one can calculate an economic payback period from an investment in a product with lower operating costs, one can also calculate an environmental payback period from expending more energy to manufacture an efficient product in order to save on operating energy. In both cases, the product must be held at least as long as the payback period in order to reap these benefits. It is often difficult for average consumers to know how long to use these products or when to replace them in order to minimize environmental impact and cost.

Using life cycle optimization, this research will prescribe when a residential air conditioning unit should be replaced with the objective of minimizing (1) energy usage, (2) green house gas emissions and (3) cost to the consumer. A model will be created to determine an optimal replacement schedule for each of these three objectives based on the climate where the unit operates. The model will also be used to examine how rates of efficiency improvement and changing energy costs impact this schedule. Then, the research will explore how demand-side management can be used to correct misalignment between the cost schedules and the energy and emissions schedules. The results will help manufacturers, consumers, and policymakers understand the environmental and economic benefits of the replacement of old air conditioners with new units.